Method and apparatus for forming latent electrostatic images

ABSTRACT

Method and apparatus for uniformly irradiating the surface of an N-type photoconductive material to decay a uniform surface charge thereon so that the surface charge potential lies on a substantially linear, steeply sloping portion of the surface charge potential-to-exposure time characteristic of the photoconductive material, thereby reducing the image exposure time.

United States Q Patent 1 Ogawa et al.

METHOD AND APPARATUS FOR FORMING LATENT ELECTROSTATIC IMAGES Inventors: Masaya Ogawa, Abeno-ku; Kuniki Seino, Amagasaki, both of Japan Assignee: Minolta Camera Kabushiki Kaisha,

Minami-ku, Osaka, Japan Filed: Apr. 17, 1972 Appl. No.: 244,984

Related U.S. Application Data Continuation of Ser. No. 850,383, Aug. 15, 1969, abandoned. I

US. Cl. 355/17, 355/3 R, 96/1 R Int. Cl G03g 13/00 Field of Search 355/3, 17;

References Cited UNITED STATES PATENTS Kaufman 355/17 X Dec. 25, 1973 Snelling 355/17 X 3,412,242 11/1968 Giaimo, Jr. 355/17 X 3,558,307 l/l971 Carlson 355/17 X 3,615,395 10/1971 Yamaji 96/l.4 3,666,364 5/1972 Marushima 355/17 X OTHER PUBLlCATlONS IBM Technical Disclosure Bulletin, Shattuck et a1. 7-66, Vol. 9, No. 2.

Primary Examiner-Richard M. Sheer Attorney-Watson, Cole, Grindle & Watson [5 7 ABSTRACT 4 Claims, 2 Drawing Figures METHOD AND APPARATUS FOR FORMING LATENT ELECTROSTATIC IMAGES This is a continuation, of application Ser. No. 850,383 filed Aug. 15, 1969, and now abandoned.

BACKGROUND OF THE INVENTION As a representative example of a known method for forming an image in electronic photographing using an N-type photoconductive material, the so-called electro-fax system making use of a zinc oxide (Zno) for an electro-photographic paper is well known.

In the zinc oxide electro-fax system, the electrophotographic paper is given a corona discharge in a dark place to give form a negative electrostatic charge on the surface thereof, and then irradiated to expose an image by the decay of the surface charge in accordance with the quantity of light received. When the difference of the surface charges reaches a maximum the exposure is discontinued to form a negative electrostatic charge latent image.

Such a phenomenon is explained in detail hereinafter with reference to FIG. 1, however, the uniform negative electrostatic charge given to the electrosensitive paper by a corona discharge device decreases in the dark portion of the image slowly at a relatively uniform speed in accordance with the exposure time. The electrostatic charge of the light portion of the image decreases slowly at first in accordance with the exposure time, and after a certain exposure time the decay factor of the electrostatic charge suddenly increases, and from that time until nearly zero potential decays relatively uniformly and rapidly. Therefore, the difference between the quantity of electrostatic charge corresponding to the dark and light image areas results in the contrast forming the electrostatic latent image on the surface of the photosensitive layer.

In such a process, at the beginning of exposure while the electrostatic charge of the light image portion of image is decaying slowly the contrast between the light and dark portions is so slight that the image exposure for this duration is not serviceable for forming the contrast of the electrostatic latent image and the image exposure time is prolonged. Further, failurein selecting the'proper exposure time results in producing an electrostatic charge latent image having a contrast not similar to the image contrast because of the change in the electrostatic charge decay rate described above.

SUMMARY OF THE INVENTION In order to eliminate the drawbacks mentioned above, in the present invention the method and device for forming an electrostaticimage are characterized in that after giving a uniform negative electrostatic charge to an electro photographic paper by a corona discharge device, the quantity of light corresponding to that necessary to change the decay rate of the electrostatic charge in the light portion of the image is irradiated uniformly all over the surface of the electrophotog'raphic paper.

The primary object of the present invention is to provide a method for forming an electrostatic image permitting-a decrease in the exposure time of the image irradiation, and making use of photo-sensitive material having powder of an'N-type photo-conductive material suchas metalic oxide as the main constituent.

The second object of the present invention is to provide an electrostatic latent image which is an accurate reproduction of an image to obtain an electronic photograph of good picture quality.

The third object of the present invention is to shorten the exposure time and enhance the duplication efficiency in an electrostatic image forming process using sensitive material.

The fourth object of the present invention is to provide a method for electrostatic image reproduction affording a reduction in the consumption of electric power by using irradiation from a light source of small capacity instead of image irradiation by a source of large capacity.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram showing the light decay of light dark image areas wherein the surface electrostatic discharge of sensitive material having powder of an N-type photo-conductive material of a zinc oxide (Zno) as the main constituents is decreased by exposure of the light and dark portions of the image.

FIG. 2 is a side view showing the disposition of essential parts of an embodiment of the device for forming an electrostatic latent image in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Prior to the explanation in detail of the embodiment of the present invention, the principle of the process by which an electrostatic latent image is formed in making use of an N-type photo-conductive material is described referring to FIG. 1. Now, in a method for forming an electrostatic latent image making use of a zinc oxide coated electro-photographic paper, when the electro-photographic paper, previously charged uniformly to the surface potential of Vs volts is exposed to image irradiation, the dark portion of the latent image has a so-called dark decay. That decay is along a gentle slope as shown by curve (a) in FIG. 1 wherein the ordinate designates the surface potential (V) and the abscissa represents the exposure time (T). The light image portion of the electro-photographic paper has a so-called light decay which has a steep slope as shown by curve (b).

And thus, the difference of surface potential (V) between curves (a) and (b) for a certain exposure time represents the contrast of the electrostatic latent image corresponding to the light and dark image portions. Therefore, when the difference in surface potential is a maximum the contrast of the electrostatic latent image is a maximum and the corresponding exposure time (t) is the proper exposure time.

However, as seen clearlyin FIG. 1 curve (b) has a slow decay at the beginning of exposure and is not greatly different from the slope of curve (a). The surface potential decreases slightly thereby making little difference between both curves (a) and (b), until however, when a certain exposure time t, is reached the decay slope becomes suddenly steep. This is a characteristic of a metal oxide especially of zinc oxide (Zno), and is explained as follows. In an N-type photoconductive material having a metal oxide such as a zinc oxide as the main constituent, when the photoconductive material is charged negatively the surface thereof attracts oxide-molecules having a negative charge. In the next step of light irradiation, electronhole pairs are emitted by the photo-sensitive layer,

, flow to the supporting body (electrode) and the photocurrent reduces the surface potential of the electrophotographic paper. .However, these oxide-molecules are strongly electronegative and repel the electrons, so that the photo-current is delayed. That is, oxidemolecules deprived of their negative charge by the aforesaid neutralization are no longer in a thermoequilibrium condition and flow from the surface of the electro-photographic paper to the air, and which is a photo induced discharge. In this case, the electrons are not released to flow to the electrode and start to decay the surface potential until the photo induced discharge is produced. The foregoing explains the decay curve as shown by curve (b) in FIG. 1.

The value exposuretime (t,) varies with the composition and constitution of; the photo-sensitive layer of zinc oxide, however, in general in so-called fax paper now in wide use the aforedescribed property is recognized almost without exception. The ratio of the exposure time (t,) to the proper time for maximum contrast may be from one-fourth to one-third.

During the exposure time (t,), as described above, the decay of surface potential is slow for both light and dark image portions, and the difference between them makes little contribution to form an image and accordingly only prolongs the image exposure time for no useful purpose. And, in the same way as conventional photography, the heavier the slope of curve (b) is to a straight line the more improved is the reproduction of the electrostatic latent image is to the image. Therefore, increasing the exposure time (t,) results in a poorer electrostatic latent image.

Taking the above .facts into consideration, in the present invention the surface potential of the electrophotographic paper is exposed up to time (t,) shown in FIG. 1 by uniformly irradiating its surface after the charge step is finished, and then the image is exposed to obtain latent image which permits the decay of potential to start from (1,).

In this manner, it is possible to shorten the image exposure time of a light source of a large capacity for projecting a general image through a series of lenses by uniform irradiation-with a light source of a small capacity which is closelyadjacent to the electrophoto'graphic paper. Therefore, it is also possible to save the consumption of electric power and prevent the large capacity light source for the image exposure from over-heating.

Then, referring to an experiment in accordance with the present invention, a known electro-fax duplicator having a xenon flash lamp as a light source for the image irradiations, two light source lamps of input 150 joules w epfit to u s e and tithe convention manner the image irradiation was produced. As a result a proper exposure of 50 lux-sec. could be given on the light image portion. Further, two light source lamps'of 100 joules input were put to use and the same image wasexposed onto the same eIe'ctro-photographic paper for the same duration, and as a result the decay of the surface potential corresponding to the light image portion was insufficient and the shortage of exposure time was indicated. However, according to the present invention a uniform irradiation corresponding to lQlux:

see. was projected oritfiri elctro-photographic paper of the same kind by a'tungsten filament lamp different from the light source used for image irradiation, and then the same image was irradiated by two light source lease at Wi le n ut for he a xposur time- As a re s ult the surface potential of electrophotographic paper corresponding to the light image portion was reduced to zero, and a duplicate having good contrast was obtained.

As described above, in the present invention prior to exposure of the image onto an electro-photographic paper uniformly negatively charged irradiation over the paper surface is projected over the paper surface by another light source and the quantity of light corresponding to exposure time (t,) is provided. By carrying this out by means of a light source closely adjacent to the electro-photographic paper it is possible to shorten the exposure time (2,). However, it is also possible that the uniform irradiation is accomplished simultaneously with the image irradiation and by doing so it is possible to shorten the exposure time much more. In this case, however, the light source providing uniform irradiation should be disposed so as not to interfere with the image irradiation.

Referring to FIG. 2, in accordance with the present invention, electro-photographic paper 1 has photosensitive layer 1' applied inuniform thickness on the paper with resin scattered with powder of a zinc oxide (Zno), and when photographic paper 1 passes through corona charge device 2 photo-sensitive layer 1' is charged uniformly with a negative charge. Tungstenfilament lamp 3 is screened by shade 4 open only in front so as to give uniform irradiation to the photosensitive layer 1 and also provide about 10 lux-sec. of light corresponding to the exposure time (t,) of the photo-sensitive layer 1'. When electro-photographic paper 1 is stopped at image irradiation position 5, manuscript 7 on transparent glass stand 6 is irradiated by light source 8 and the image thereof is formed on photosensitive layer 1' by means of mirror 9 and lens 10. Light source 8 provides about 40 lux-sec.-- -corresponding to (t, t) mentioned above--to photosensitive layer l' While passing through charge device 2 and the front of uniform irradiation light source 3, electrophotographic paper 1 respectively receives a uniform charge step and a uniform irradiation asit moves. Subsequently, the paper is stopped to be exposed to the image irradiation. However the electro-photographic paper 1 may travel while being exposed by arranging to move manuscript 7 on transparent glass stand 6 in synchronization with electro-photographic paper 1. Thus, it is possible to provide a completely moving system as well.

.In the embodiment of the present invention, only a light source for uniformlyirradiating the photosensitive metal oxide to the surface of said photoconductive material and thereby creating electron-hole pairs therein,

uniformly irradiating said photoconductive material from a radiation source facing said photoconductive material thereby causing neutralization of the holes in said electron-hole pairs emitted by said photoconductive layer by said negatively charged metal oxide molecules to release electrons for neutralizing the surface potential of said electroconduetive medium,

exposing an image onto the surface of said photoconductive material, and

said photoconductive material has a surface charge potential-to-exposure time characteristic in which the decay of surface potential substantially increases after a given exposure time T and said uniform irradiation decays the surface potential of said photoconductive material to that surface potential corresponding to exposure time T 2 A method as in claim 1 wherein said surface charge potential-to-exposure time characteristic is substantially linear from said time T to a time T corresponding to zero surface potential, and the duration of said uniform irradiation is greater than T and less than T 3. A method as in claim 1 wherein said photoconductive material comprises zinc oxide, the level of uniform irradiation of the surface of the photoconductive material is approximately l lux-secs., and the image is exposed onto the surface of the photoconductive material with-an irradiation level of approximately 40 lux-secs.

4. Apparatus for forming a latent electrostatic image on an electroconductive medium having a surface thereof coated with an N-type photoconductive material including a metal oxide, comprising;

means for uniformly charging said photoconductive material comprising a paper-like base having a surface coated with an N-type photoconductive material including a metal oxide with a negative charge to attract the negatively charged molecules of said metal oxide to the surface of said photoconductive material and creating electron-hole pairs therein;

means for uniformly irradiating said photoconductive said photoconductive material has a surface charge potential-to-exposure time characteristic in which the decay of surface potential substantially increases after a given exposure time T and said uniform irradiation decays the surface potential of said photoconductive material to that surface potential corresponding to exposure time T means for exposing an image onto the surface of said photoconductive material, and

means for successively transporting said photoconductive material to the aforesaid means.

Attesting Officer UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent I Io. 327812108 Dated December 25, 1973 Inventofls) "Masaya Ogawa et a1 .It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

[30] Foreign Application Priority Date August: 28, 1968 Japan No. 61687/1968 Signed an d sealed this 11th day or Junie-m.

(SEAL) Attest: I

EDWARD M.FLETCHER,JR. c. MARSHALL DANN Commissioner of Patents RM P0-\0 0 USCOMM-DC 60376-P69 I U.$. GOVERNMENT PRINTING OFFICE: [9C9 0-866-33,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, ,7 1,1 Dated December 25,

I nventor(s) "Ma aya Ogawa et a1 .It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below;

I I [30] Foreign Application Priority Date August 28, 1968 Japan No. 61687/1968 Signed sealed this llth day of June 1971]..

(SEAL) Attestr C EDWARD M.FLE.'I'CHER,JR. 'c. MARSHALL DANN Attesting Officer Commissioner of Patents FORM PO-IO O (1 uscoMM-Dc 60376-P69 U. 5. GOVERNMENT PRINTING OFFICE ISIS O36G-334, 

1. A method for forming a latent electrostatic image on photoconductive material, comprising the steps of: uniformly charging said photoconductive material comprising a paper-like base having a surface coated with an N-type photoconductive material including a metal oxide with a negative charge to attract the negatively charged molecules of said metal oxide to the surface of said photoconductive material and thereby creating electron-hole pairs therein, uniformly irradiating said photoconductive material from a radiation source facing said photoconductive material thereby causing neutralization of the holes in said electron-hole pairs emitted by said photoconductive layer by said negatively charged metal oxide molecules to release electrons for neutralizing the surface potential of said electroconductive medium, exposing an image onto the surface of said photoconductive material, and said photoconductive material has a surface charge potential-toexposure time characteristic in which the decay of surface potential substantially increases after a given exposure time T1 and said uniform irradiation decays the surface potential of said photoconductive material to that surface potential corresponding to exposure time T1.
 2. A method as in claim 1 wherein said surface charge potential-to-exposure time characteristic is substantially linear from said time T1 to a time T0, corresponding to zero surface potential, and the duration of said uniform irradiation is greater than T1 and less than T0.
 3. A method as iN claim 1 wherein said photoconductive material comprises zinc oxide, the level of uniform irradiation of the surface of the photoconductive material is approximately 10 lux-secs., and the image is exposed onto the surface of the photoconductive material with an irradiation level of approximately 40 lux-secs.
 4. Apparatus for forming a latent electrostatic image on an electroconductive medium having a surface thereof coated with an N-type photoconductive material including a metal oxide, comprising; means for uniformly charging said photoconductive material comprising a paper-like base having a surface coated with an N-type photoconductive material including a metal oxide with a negative charge to attract the negatively charged molecules of said metal oxide to the surface of said photoconductive material and creating electron-hole pairs therein; means for uniformly irradiating said photoconductive material, thereby causing neutralization of the holes in said electron-hole pairs emitted by said photoconductive layer by said negatively charged metal oxide molecules to release electrons for neutralizing the surface potential of said electroconductive medium, said irradiating means are disposed so as to face said photoconductive material, said photoconductive material has a surface charge potential-to-exposure time characteristic in which the decay of surface potential substantially increases after a given exposure time T1 and said uniform irradiation decays the surface potential of said photoconductive material to that surface potential corresponding to exposure time T1, means for exposing an image onto the surface of said photoconductive material, and means for successively transporting said photo-conductive material to the aforesaid means. 